Mechanistic Study on the Bis(p-sulfonatophenyl)phenylphosphine Synthesis of MonometallicPt Hollow Nanoboxes Using Ag*-Pt Core-Shell Nanocubes as Sacrificial Templates

Yen-Nee Tan,‡ Jun Yang,† Jim Yang Lee,*,†,‡ and Daniel I. C. Wang‡,§

Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent RidgeCrescent, Singapore 119260, Singapore-MIT Alliance, National UniVersity of Singapore, 4 EngineeringDriVe 3, Singapore 117576, and Department of Chemical Engineering, Massachusetts Institute of Technology,Room 16-429, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139

ReceiVed: May 28, 2007; In Final Form: July 25, 2007

We report herewith a simple and effective method for the preparation of anisotropic Ag*-Pt core-shellnanocubes and Pt nanoboxes. The core-shell nanocubes were first synthesized by the coreduction of Ag+

and PtCl62- and then treated in a bis-(p-sulfonatophenyl)-phenylphosphine (BSPP) solution to remove thecore materials. We found that BSPP, in addition to being an effective silver oxidant, was also a good solubilizerfor AgCl nanoparticles at room temperature. This allowed us to prepare phase-pure Pt nanoboxes from theas-synthesized Ag*-Pt nanocubes using a greatly simplified posttreatment for AgCl, a reaction byproductand known impurity in the synthesis of hollow nanostructures by the replacement reaction approach (Ag* isused here to denote nanocomposites of Ag and AgCl). We also showed that AgCl functioned as a removaltemplate in enabling a shape-controlled synthesis of the nanostructures.

Introduction

After close to a decade of concerted effort, many metalnanoparticles can now be prepared with a fairly good controlof shapes and sizes. A number of geometries including wires,1

rods,2 cubes,3 and prisms4 can also be generated by solutionchemistry methods. The interest now shifts to the synthesis ofmore complex structures such as core-shell and hollow particlesfor applications in catalysis,5 chemical and biological sensors,6

and surface-enhanced Raman scattering spectroscopy.2,7 Whilea number of core-shell and hollow nanospheres have beenfabricated, the morphology control of such complex nanostruc-tures has rarely been satisfactory.

The Xia group has pioneered a method to fabricate hollowmetal particles based on galvanic replacement reactions carriedout in the aqueous phase.8 They demonstrated the method withthe synthesis of hollow Au/Ag nanostructures using Ag nanocubesas the sacrificial templates to reduce an aqueous HAuCl4

solution. Depending on the experimental conditions, eithersmooth alloy nanoshells or monometallic Au nanoshells withpinholes were formed. The extension of this method to thepreparation of Pt/Ag nanoboxes using sacrificial Ag nanocubetemplates and aqueous Na2PtCl4 solution was, however, unsuc-cessful.9 The nanoboxes formed were Pt shells with a bumpyexterior and some residual Ag in the interior, and would collapseinto aggregates of Pt nanoparticles if all the Ag was replaced.The preparation of monometallic Pt nanoboxes with a smoothexterior therefore remains a challenge. Recently, our groupreported a bis-(p-sulfonatophenyl)phenylphosphine (BSPP)-based synthesis of Pt hollow nanospheres, which was more ableto preserve the template geometry and to control the shellcomposition.11 The method begins with the preparation of

bimetallic core-shell spherical Ag-Pt nanoparticles by thesuccessive reduction of Ag and Pt precursor salts, and isfollowed by the selective dissolution of the Ag cores with BSPP.The hollow Pt nanoparticles prepared as such retained thespherical geometry and had nearly the same size as their core-shell predecessors. We are naturally very interested in extendingthe utility of this approach to nonspherical particles.

The preparation of core-shell Ag-Pt nanocubes is animportant step preceding the preparation of Pt hollow nanoboxes.We used a simple, one step synthesis to prepare Ag*-Pt core-shell nanocubes by the coreduction of Ag and Pt precursor saltswith sodium citrate, instead of the usual practice of a two-stepsuccessive reduction involving a strong reducing agent such asNaBH4 or hydrazine (Ag* is used here to denote nanocompositesof Ag and AgCl).12 The reduction kinetics was more controllableby this means, thereby promoting the formation of anisotropicshapes. The most interesting finding is that the AgCl nanopar-ticles formed upon the mixing of the two metal precursor saltswas crucial to the formation of Ag*-Pt core-shell nanocubes.A systematic study of the role of AgCl in the growth and controlof the final geometry of the Ag-Pt nanostructures was thencarried out. After the successful preparation of core-shellnanocubes, hollow Pt nanoboxes were then obtained by etchingout the silver-containing cores with BSPP. This method ofpreparation has several advantages compared to a typicalreplacement reaction approach: (1) The BSPP synthesis canbe carried out at room temperature with a greater ease of control.On the contrary, the replacement reaction approach requireselevated temperatures. (2) The replacement reaction is morepredisposed to forming alloy nanoshells,8-9 whereas the BSPPsynthesis forms only pure Pt nanoshells. In addition, as will bedescribed in detail later, the dual function of BSPP as a Agoxidizer and a AgCl solubilizer, which was hitherto unknown,circumvented the problem of AgCl contamination in the productthat has long beset the replacement reaction synthesis of hollowstructures using sacrificial Ag nanoparticle templates.8-9

† Department of Chemical and Biomolecular Engineering, NationalUniversity of Singapore.

‡ Singapore-MIT Alliance, National University of Singapore.§ Massachusetts Institute of Technology.

14084 J. Phys. Chem. C2007,111,14084-14090

10.1021/jp0741049 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 09/06/2007

Experimental Section

Materials. AgNO3 (99.8%) and H2PtCl6 (99.9%) fromAldrich, sodium citrate (98%) from Merck, and BSPP dihydratedipotassium salt (97%) from Strem Chemicals were used asreceived. Deionized water was filtered by a Milli-Q waterpurification system. All glassware and Teflon-coated magneticstir bars were cleaned in aqua regia, followed by copious rinsingwith deionized water before drying in an oven.

Synthesis of Ag*-Pt Core-Shell Nanocubes. In a typicalsynthesis, 10 mL of 1 mM H2PtCl6 aqueous solution was mixedwith 10 mL of 1 mM AgNO3 aqueous solution at roomtemperature. The mixture was refluxed at 100°C for 5 min inan oil bath before adding 2 mL of 40 mM trisodium citratesolution quickly. Reflux was continued for another 4 h toproduce a yellowish brown hydrosol as the final product.

Synthesis of Pt Nanoboxes.15 mg of solid K2BSPP wasadded to 5 mL of the as-synthesized bimetallic nanocubesprepared above. The mixture was aged for more than 3 h tocomplete the dissolution of Ag and AgCl. No color changes ofthe yellowish brown hydrosol was observed upon the additionof K2BSPP and thereafter.

Characterizations.A JEOL JEM2010 microscope was usedto obtain TEM images of the nanoparticles. For TEM measure-ments, a drop of the nanoparticle solution was dispensed ontoa 3 mm copper grid covered with a continuous carbon film.Excess solution was removed with an adsorbent paper. Thesample was dried under vacuum at room temperature. Thenanoparticles before and after BSPP treatment were analyzedby energy-dispersive X-ray (EDX) on a JEOL MP5600 scanningelectron microscope (SEM) and by X-ray photoelectron spec-troscopy (XPS) on a VG ESCALAB MKII spectrometer.Powder X-ray diffraction measurements were performed on aBruker D8 Advanced diffractometer (Cu KR radiation, λ )1.5418 Å), operating at 40 mA and 40 kV. Samples were drawnfrom the nanoparticles spun down in a centrifuge, which werewashed copiously with deionized water and vacuum-dried atroom temperature.

Results and Discussion

Synthesis and Characterizations of Ag*-Pt Core-ShellNanocubes and Pt Nanoboxes. Ag*-Pt core-shell nanocubeswere produced by the simultaneous reduction of Ag and Ptprecursor salts using a mild reducing agent, sodium citrate. Thereduction kinetics was controlled to promote the formation ofthe anisotropic shape. These bimetallic nanostructures weresubsequently used to form Pt nanoboxes by the BSPP etchingof the silver-containing cores.

Figure 1 shows the representative electron microscope imagesof the Ag*-Pt core-shell nanocubes and Pt nanoboxes atrelatively low magnifications. Over 95% of the hollow structures(Figure 1c,d) had the same morphology as the starting cubictemplates (Figure 1a,b). This shows that BSPP served not onlyas the etchant to remove the Ag-containing cores but also asthe substitute stabilizer for the initially citrate-stabilized core-shell nanocubes. In addition, the removal of the cores from theAg*-Pt core-shell nanocubes did not cause the collapse ofthe cubic geometry.

The core-shell architecture of the Ag*-Pt core-shellnanocubes was revealed by high-resolution TEM imaging. Therewere distinguishable brightness differences between the innerand outer regions of the nanocubes, as shown in Figure 2a.Additional structural details were revealed by lattice fringes witha spacing of 0.233 nm in the core area. However, thesuperimposition of interference patterns from two highly similar

face-centered cubic (fcc) metals (d(111) for Ag and Pt are0.2356 and 0.2255 nm, respectively) made it difficult todifferentiate between Ag and Pt in the HRTEM image. Thus, asimple method based on the selective oxidation of Ag by BSPPwas used to confirm the core-shell structure of the bimetallicnanocubes.11 A comparison of the TEM images of Ag*-Ptcore-shell nanocubes before (Figure 1a) and after (Figure 1c)BSPP oxidation showed only the removal of the cores, indicatingthe core and shell components of the nanocubes were Ag* andPt, respectively. The high-resolution TEM (HRTEM) image inFigure 2b shows the discontinuous shell of Pt nanoparticles thathad initially decorated the now-removed Ag-containing core.The existence of a discontinuous Pt shell on the Ag-containingcore was important, as it allowed BSPP penetration to oxidizethe underlying Ag-containing core.

The composition of the bimetallic nanoparticles was analyzedby EDX and XPS. Depending on the concentrations of the metalprecursors, a white solid appeared at times at the bottom of thereaction vessel when the reaction mixture was cooled to roomtemperature. This was due to the precipitation of AgCl, thesolubility of which is lower at reduced temperatures. To preventthe AgCl precipitate from interfering with the compositionanalysis, the bimetallic nanocubes were in this case treated withsupersaturated NaCl solution to solubilize AgCl through thecomplex formation reaction shown below:13

Ag*-Pt Nanocubes.The nanocubes after the NaCl treatmentwere recovered by centrifugation (10 000 rpm for 30 min)

Figure 1. (a) TEM and (b) SEM images of Ag*-Pt core-shellnanocubes; (c) TEM and (d) SEM images of Pt nanoboxes. (Note: Ag*represents the Ag (AgCl) nanocomposite).

Figure 2. HRTEM images of (a) Ag*-Pt core-shell nanocubes and(b) Pt nanoboxes with a discontinuous shell. The upper right insetsshow the SEM images of an individual particle while the lower rightinsets show the lattice fringes at the center of the nanoparticle.

AgCl(s) + 3Cl-(aq)f AgCl43-(aq) (1)

BSPP Synthesis of Monometallic Pt Hollow Nanoboxes J. Phys. Chem. C, Vol. 111, No. 38, 200714085

followed by several washes before they were redispersed inwater for subsequent analyses. The EDX spectrum in Figure3a shows emission peaks typical of Pt and Ag, from which aAg to Pt atomic ratio of 0.3:0.7 was calculated. On the contrary,almost no Ag presence was detected by XPS, indicating thatthe surface of the bimetallic nanoparticles was Pt rich. Thedifference in elemental compositions between XPS (a surfaceanalysis technique) and EDX (a bulk analysis technique)measurements is strong evidence for the presence of a Pt shellon the Ag-containing core. EDX also detected a noticeableamount of Cl (Figure 3a) while XPS analysis indicated none.This rules out the possibility of AgCl adsorption on the nanocubesurface or its presence as an extraneous impurity in the solutionphase (because the nanocubes were purified with supersaturatedNaCl solution). AgCl was therefore part of the core component.

Pt Nanoboxes.The BSPP-treated nanocubes showing ahollow interior were also analyzed by EDX. The EDX spectrumwas dominated by the Pt signal with several Cu peaks from theTEM grid, and no Ag (Figure 3b). Electron diffraction and XRDmeasurements were also carried out to confirm the crystal-lographic information in small and large samples. The diffractionpeaks in the XRD pattern of Figure 4 were found to be broad,which is normally the case for small nanoparticles. The shellsof the hollow nanoboxes were polycrystalline, as confirmed bythe HRTEM image, which shows multiple crystallographicdomains (Figure 2c). Additional evidence was obtained fromelectron diffraction (ED) that shows several bright continuousconcentric rings (inset of Figure 4) attributable to the diffractionsfrom the{111}, {200}, {220}, {311}, and{420} planes of fccPt.14a Thus monometallic Pt nanoboxes were formed from theAg*-Pt core-shell nanocubes after the BSPP treatment. The

results here are good evidence to show that BSPP served notonly as a silver oxidizer, but also as a AgCl solubilizer (seebelow).

BSPP as Ag Oxidant cum AgCl Solubilizer.A simplecontrol experiment was carried out to confirm the solubilizingaction of BSPP. A AgCl nanoparticle suspension (see experi-mental details in Supporting Information) was divided into twobatches and added with BSPP and saturated NaCl, respectively.Immediate color change from cloudy yellow to colorless wasobserved in both cases. This shows that BSPP was able tosolubilize AgCl as good as saturated NaCl, if not better. Inaddition, TEM imaging of samples drawn from either of thecolorless solutions showed no presence of particles.

The mechanism of BSPP solubilization is similar to that ofNaCl. It is believed that BSPP chelated with the silver ions,forming BSPP-Ag+ complexes that disrupted the followingequilibrium of AgCl in solution:

The decrease in the free silver ion concentration in thesolution due to the formation of BSPP-Ag+ complexes shiftedthe equilibrium to the left, resulting in the dissolution of AgClnanoparticles (See Supporting Information for the details of asubstantiation experiment).

The Hidden Role of AgCl in Templating the Formationof Ag*-Pt Nanocubes.It is well known that mixing a solutionof Ag+ with a solution of Cl- would almost quantitativelyprecipitate the chloride ions as solid silver chloride (AgCl), dueto the low solubility product of silver chloride at roomtemperature (ksp ) 1.82× 10-10, T ) 20 °C).16 Hence, in thepreparation of bimetallic Ag-Pt nanoparticles using an aqueousmixture of metal precursor salts in the form of H2PtCl6 andAgNO3, one could not rule out a priori the precipitation of AgClbefore the addition of the reducing agent to the metal precursormixture. The formation and morphology of the bimetallicnanoparticles could be significantly affected by the presenceof the AgCl preparticles.

Formation of AgCl Preparticles upon Mixing of MetalPrecursors before Chemical Reduction.Ten minutes of cen-trifugation of the mixture of the metal precursor salts at 10 000rpm resulted in the settlement of a cloudy yellowish precipitate,which was recovered, cleansed, and examined by TEM. Figure5a shows that the recovered nanoparticles were cubic in shape.The ED and XRD patterns of the precipitate showed a strongpresence of AgCl nanocrystals (Figure 6). When compared withthe literature values14b,cgiven in Table 1, the detection of someAg electron diffraction patterns could be consequential uponthe electron beam reduction of AgCl to Ag. Supporting evidencewas provided by EDX analysis of the nanoparticles (Figure 5c).It was noted that under the exposure of the electron beam inEDX analysis, there was a rapid decrease of the chlorine signal,while the silver signal remained practically unchanged. This

Figure 3. EDX analyses of core-shell Ag*-Pt nanoparticles before (a) and after (b) BSPP treatment.

Figure 4. X-ray diffraction pattern of Pt nanoboxes. Inset is the EDpattern taken from a nanobox showing several bright concentric ringscorresponding to the diffraction of fcc Pt.

Ag+(aq)+ Cl-(aq)T AgCl(s) (2)

14086 J. Phys. Chem. C, Vol. 111, No. 38, 2007 Tan et al.

shows that the AgCl preparticles were unstable and easilyreduced to Ag by the electron beam.15

On the other hand, when the supernatant after the removalof the AgCl precipitate was reduced by sodium citrate at boilingtemperature, TEM examination of the resulting product indicatedthe presence of very small spherical nanoparticles (Figure 5b).EDX analysis determined these nanoparticles as monometallic

Pt with no detectable trace of Ag or Cl (Figure 5d). Hence,most of the silver ions had been precipitated as silver chloridepreparticles at room temperature and were removed from thereaction mixture before chemical reduction by sodium citrate.

Effect of AgCl Preparticles on the Formation of CubicNanoparticles during Reduction Reaction.The nucleation ofmetallic clusters and their subsequent growth into nanocrystals

Figure 5. TEM images of (a) AgCl nanoparticles precipitated from the reaction mixture before reduction by sodium citrate; (b) Pt nanoparticlesobtained from the reduction of supernatant by sodium citrate; EDX analyses of (c) AgCl nanocubes and (d) Pt nanoparticles corresponding to theTEM images in (a) and (b), respectively.

Figure 6. (a) Electron (ED) and (b) X-ray (XRD) diffraction patterns of AgCl precipitated from the reaction mixture before citrate reduction.

TABLE 1: Major X-ray Diffraction Patterns of AgCl and Ag 14b,c

plane (hkl)

literature XRDdata for AgCl

d (nm)

literature XRDdata for Ag

d (nm) spot number

experimental datafrom the ED ringsof AgCl nanocubes

d (nm)

111 0.320 0.236 1 0.324200 0.277 0.204 2 0.280220 0.196 0.145 3 0.238311 0.167 0.123 4 0.200400 0.139 0.102 5 0.164420 0.124 0.092 6 0.141

BSPP Synthesis of Monometallic Pt Hollow Nanoboxes J. Phys. Chem. C, Vol. 111, No. 38, 200714087

with a particular morphology are strongly influenced by thepresence of a foreign surface, the AgCl preparticles in this case.Although the solubility product of AgCl is known to increasewith temperature, reaching a calculated value ofksp ) 5.85×10-8 at 100°C (using∆H0 ) 65.57 kJ/mol),16 the multiplicationproduct of concentrations of [Ag+][Cl-] (Qsp) in our experimentwas still orders of magnitude higher (1.24× 10-6), and hencemost of the AgCl preparticles were not soluble at the reactiontemperature of 100°C and were present as a foreign surface totemplate the morphosynthesis of bimetallic nanoparticles. Acontrol experiment was set up using metal precursor concentra-tions that were five times lower to ensure the full dissolutionof AgCl at the reaction temperature. The result was no cubicnanoparticle formation in the absence of the AgCl hard templates(Figure 7d). Additional experiments were carried out by usingdifferent concentrations of the metal precursor salts to alter theQsp values. The TEM images in Figure 7 show the evolution ofshapes of the bimetallic nanostructures from nanocubes withdistinct edges to spherical nanoparticles with decreasingR )Qsp/ksp (T ) 100°C) values. WhenR > 1, AgCl was insolubleand was available to template the formation of nanocubes duringthe reduction reaction. This contrasted starkly with the case ofR < 1, where only spherical nanostructures were producedbecause of the lack of a templating surface. This understandingalso rationalizes the results of a previous study on the synthesisof Ag-Pt bimetallic nanoparticles in water-in-oil microemul-sions, whereby the formation of “undesirable” AgCl precipitatewas suppressed via the displacement reaction of “Na-AOT +Ag+ f Na+ + Ag-AOT”, resulting in the exclusive formationof spherical nanoparticles.12c

Proposed Mechanism for the Formation of Ag*-PtCore-Shell Nanocubes and Pt Nanoboxes.Ag*-Pt core-shell nanocubes were obtained by coreducing a mixture of equalvolumes of 1 mM AgNO3 and 1 mM H2PtCl6 aqueous solutionswith sodium citrate at 100°C. From the previous discussion, itis known that AgCl precipitation would occur and form cubic-shaped preparticles in the reaction mixture before chemicalreduction. Experimental results revealed the hidden role of thesepreparticles in templating the formation of bimetallic nanocubesduring reduction. The following equations describe the spon-taneous formation of AgCl nanoparticles upon mixing thesolutions of the metal precursor salts and the subsequentformation of elemental silver upon exposure to light or in thepresence of a reducing agent (sodium citrate):

Hydrolysis of H2PtCl617

Formation of AgCl

Reduction of AgCl

In the above reactions, the hydrolysis of H2PtCl6 initiatedthe formation of AgCl nanoparticles. H2PtCl6 as an acid isknown to undergo hydrolysis resulting in the release of freechloride ions.17 Although the cubic geometry is common forAgCl nanoparticles formed by the double jet method,18 whereAgNO3 and KCl aqueous solutions were delivered simulta-neously to a KCl solution with gelatin at a constant rate, it wassurprising to find that cubic AgCl nanoparticles could also beformed by simply mixing H2PtCl6 and AgNO3 solutions. It isbelieved that the controlled release of Cl- ions from H2PtCl6hydrolysis reaction was the key to the formation of theanisotropic AgCl particles.

Figure 8 provides an overview of the major processesinvolved in the formation of Ag*-Pt core-shell nanocubes andPt nanoboxes. The presence of AgCl preparticles was essentialto the formation of bimetallic nanostructures with retention ofthe cubic geometry during chemical reduction. The reductionof AgCl in the solid form by a reducing agent has been knownfor a long time.19 It is suggested that the reaction probablystarted at some defective surface sites or surface heterogeneitiesand was autocatalytic at the Ag-AgCl interfaces, resulting ina rapid expansion of the Ag domains. The autocatalytic natureof the reaction thus allowed the AgCl core to be covered quicklywith Ag, thereby retaining the cubic geometry. When theaccumulation of Ag atoms on the template surfaces wassufficient to provide a complete monolayer coverage, theunderlying AgCl template was insulated from further reaction,ceasing the growth of the Ag (AgCl) nanocomposite corestructure. EDX analysis (Figure 3a) provided the evidence insupport of the formation of Ag (AgCl) nanocore: the measuredatomic ratio of Ag:Cl) 0.55:0.45 shows Ag enrichment ofthe structure resulting from the surface reduction of AgCl toAg with concomitant release of Cl- to the environment.

In a bimetallic system prepared by the simultaneous reductionof two metal ions, the ions with the more positive redox potentialare generally reduced first. The standard redox potentials (E)(versus reversible hydrogen electrode) of Ag+ and PtCl62- aregiven below:12c

The single-electron reduction of Ag+ should be kineticallymore facile than the multiple-electron reduction of PtCl6

2-,which has to proceed through a number of single-electron

Figure 7. TEM images of the reaction products obtained at differentmetal precursor concentrations (Qsp ) product of maximum concentra-tions of Ag and Cl ions in the reaction solution;R ) Qsp/ksp,T ) 100 °C): (a) R ) 30; (b) R ) 20; (c) R ) 10, and (d)R ) 0.8.

[PtCl6]2- + xH2O f [PtCl6-x(H2O)x]

(2-x)- + xCl- (3)

[PtCl6-x(H2O)x](2-x)- + yOH- f

[PtCl6-x(OH)y(H2O)x-y](2-x+y)- + yH2O (4)

[PtCl6-x(H2O)x](2-x)- + zOH- f

[PtCl6-x-z(OH)z(H2O)x](2-x+z)- + zCl- (5)

Ag+(aq)+ Cl-(aq)f AgCl(s) (6)

AgCl(s) + e y\zreductant

Ag(s) + Cl-(aq) (7)

Ag+ + e- f Ag0; E0 ) +0.80 V (8)

PtCl6-2 + 4e- f Pt0 + 6Cl-; E0 ) +0.74 V (9)

14088 J. Phys. Chem. C, Vol. 111, No. 38, 2007 Tan et al.

reactions. The preferential deposition of a silver layer on thetemplate surface before the deposition of Pt nanoshells couldtherefore be understood. It should be mentioned that Pt doesnot readily under go solid-solid diffusion with Ag at temper-atures below 900 K, thus there was no possibility for alloynanoshell formation in this case.10

Under the prevailing experimental condition in this study,not all the Ag+ would precipitate as AgCl. The proposedmechanism implicitly assumes that the reduction of the free Ag+

in the solution phase occurred at a slower rate than theheterogeneous autocatalytic reduction of AgCl. Because of thehigh Pt concentration in the solution, the diffusion of Pt atomsto the solid surface occurred at a faster rate than any would-beAg atoms, resulting in the deposition of a Pt nanoshell on theexterior of the Ag (AgCl) nanocore. The formation of the Ag(AgCl) core Pt-shell nanocubes was then completed. We haveobserved in our previous study that the reduction of Ag+ in thepresence of Pt nanoparticles would not result in any core-shellproduct. Instead, a physical mixture of Ag nanoparticles andthe original Pt seeds was obtained.20 Hence, once the Ag (AgCl)core-Pt shell structure was established, the Ag atoms in thesolution formed upon reduction would nucleate and grow as aseparate phase instead of deposition on the Pt shell. Because oftheir small size, the Ag nanoparticles formed remained in thesolution when the Ag (AgCl) core-Pt shell nanocubes werecollected by centrifugal separation.

The successful preparation of the Ag*-Pt core-shellnanocubes enabled the synthesis of Pt nanoboxes using BSPPas the etchant. Previous work has shown BSPP as a good silveretchant that can selectively oxidize the Ag core in core-shellnanoparticles.11 In this work, we discovered that BSPP not onlyoxidized Ag nanoparticles but also solubilized AgCl nanopar-ticles. In the synthesis of Pt nanoboxes, the BSPP added to theAg*-Pt core-shell nanocubes would first diffuse through thediscontinuous Pt shell to oxidize the silver layer on the AgCl

template, thereby exposing the underlying AgCl structure forfurther reaction. The formation of Ag-BSPP complexes wouldthen cause the dissolution of the AgCl cores resulting in theformation of Pt nanostructure with large cavities, that is, Ptnanoboxes. While the Ag layer on the AgCl nanoparticlesresisted dissolution by saturated NaCl solution, BSPP was aneffective two-in-one etchant that could completely hollow outthe core materials in the formation of phase pure Pt-nanoboxes.

Conclusion

In summary, we have demonstrated a simple and effectiveroute for the preparation of anisotropic Ag*-Pt core-shellnanocubes and Pt nanoboxes. The core-shell nanocubes werefirst synthesized and the discontinuous Pt shell on the silver-containing core allowed BSPP to diffuse through the shell todissolve the underlying core materials, leaving a Pt shell withthe cubic geometry intact. This new synthesis method usingcore-shell nanocubes as a sacrificial template in the formationof nanoboxes has several notable advantages: (1) it is suitablefor the synthesis of monometallic nanoshells instead of the morecommon alloy nanoshells, (2) preservation of the templategeometry, and (3) the synthesis can be carried out at roomtemperature and with a greater ease of control. This methodleverages heavily on the bifunctionality of BSPP as a Agoxidizer and a AgCl solubilizer. This new process addressesthe problem of AgCl contamination in the common replacementreaction synthesis of hollow structures using sacrificial Agnanoparticle templates.

The preparation of core-shell Ag*-Pt nanocubes was animportant step preceding the preparation of Pt hollow nanoboxes.This study also revealed the hidden role of AgCl in transferringthe template geometry to the product. Two distinctive steps wereinvolved: first the formation of templating AgCl nanocubesthrough the reaction between Ag+ and Cl- released from the

Figure 8. Schematic illustration outlining the proposed models on the syntheses of Ag (AgCl)-Pt core-shell nanocubes and Pt nanoboxes: (a)Direct attack of the reducing agent on the active sites of the preformed silver chloride surfaces. (b) and (c) Autocatalytic growth of the silverdomains. (d) Establishment of a silver layer on the surface of the silver chloride template. (e) Deposition of Pt atoms atop the silver layer formingthe cubic Pt nanoshell. (f) Penetration of BSPP molecules through the discontinuities in the Pt shell to etch out the silver-containing core. (g)Formation of Pt nanoboxes. The cross-sectional views of the plane dissected by the dashed lines are shown below each Figure.

BSPP Synthesis of Monometallic Pt Hollow Nanoboxes J. Phys. Chem. C, Vol. 111, No. 38, 200714089

spontaneous hydrolysis of H2PtCl6. The Ag+ and Cl- concentra-tions in the solution had to be high enough to resist thedissolution of AgCl template at the reaction temperature. In thesecond step of chemical reduction by sodium citrate at 100°C,the direct attack of citrate on the active sites of the silver chloridesurface caused the latter to be covered with an impervious layerof Ag that insulated the underlying AgCl from further reaction.Subsequent reduction of the Pt ions in the solution-phase wouldthen deposit a Pt overlayer on the Ag (AgCl) surface to formAg*-Pt core-shell nanocubes.

Acknowledgment. This work was supported by Singapore-MIT Alliance. Y.-N.T. would like to acknowledge Singapore-MIT alliance for her research scholarship.

Supporting Information Available: Evidence of BSPP-assisted dissolution of AgCl nanoparticles. Confirmation ofBSPP-Ag complex formation. This material is available freeof charge via the Internet at http://pubs.acs.org.

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